There are certainly worse items, but he's paying a lot of money for HP when he already has one of the best speed modifiers in the game at his disposal (Q), so the slow is essentially wasted. I'd rather have items that give MR + HP or Armor + HP like Randuin's, Banshee's, etc. I also enjoy Glacial Shroud for the chunk of CDR and mana which feeds Q spam.

Frozen Mallet is good on champs that can close the distance, but not keep an enemy in range to seal the deal.

One intelligent means of using it on Malphite would be effectively slowing two targets at once, by using Q on one, and auto-attacking the other.

Its an awful item on him really. He has barely any melee range and if hes not using Q he cant keep up with them to do the hits and Q slows them down anyway. Health isnt a hugely great issue with malph as you get a small amount on BV and a nice amount on Sunfire. The rest is ARM and MR which is better for him due to the huge amounts of items and abilities that take % heatlh.

thanks guys i mean his reasoning behind it was that he was fed so the situation allowed it. of course the situation allows it cuz you can build ANY item on someone when they are fed. you could put a sunfire on ashe doesnt mean its a good item for her. idk he is just stubborn so i wanted to shut him up with this forum lol